## Autograd and the derivative of an integral function

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There are many functions that are defined by integrals. The error function, for example is defined by $$erf(x) = \frac{2}{\sqrt{\pi}}\int_0^x e^{-t^2}dt$$.

Another example is:

$$\phi(\alpha) = \int_0^1 \frac{\alpha}{x^2 + \alpha^2} dx$$.

We have reasonable ways to evaluate these functions numerically, e.g. scipy.integrate.quad, or numpy.trapz, but what about the derivatives of these functions? The analytical way to do this is to use the Leibniz rule, which involves integrating a derivative and evaluating it at the limits. For some functions, this may also lead to new integrals you have to numerically evaluate. Today, we consider the role that automatic differentiation can play in this.

The idea is simple, we define a function in Python as usual, and in the function body calculate the integral in a program. Then we use autograd to get the derivative of the function.

In this case, we have an analytical derivative to compare the answers to:

$$\frac{d\phi}{d\alpha} = -\frac{1}{1 + \alpha^2}$$.

## 1 Example 1

For simplicity, I am going to approximate the integral with the trapezoid method in vectorized form. Here is our program to define $$\phi(\alpha)$$. I found we need a pretty dense grid on the x value so that we have a pretty accurate integral, especially near $$x=0$$ where there is a singularity as α goes to zero. That doesn't worry me too much, there are better integral approximations to use, including Simpson's method, adaptive methods and perhaps quadrature. If you define them so autograd can use them, they should all work. I chose the trapezoidal method because it is simple to implement here. Note, however, the autograd.numpy wrappers don't have a definition for numpy.trapz to use it directly. You could add one, or just do this.

import autograd.numpy as np

def trapz(y, x):
d = np.diff(x)
return np.sum((y[0:-1] + y[1:]) * d / 2)

def phi(alpha):
x = np.linspace(0, 1, 1000)
y = alpha / (x**2 + alpha**2)
return trapz(y, x)

# This is the derivative here!


Now, we can plot the derivatives. I will plot both the analytical and automatic differentiated results.

%matplotlib inline
import matplotlib.pyplot as plt

alpha = np.linspace(0.01, 1)

# The AD derivative function is not vectorized, so we use this list comprehension.
dphidalpha = [adphi(a) for a in alpha]

def analytical_dphi(alpha):
return -1 / (1 + alpha**2)

plt.plot(alpha, analytical_dphi(alpha), label='analytical')
plt.xlabel(r'$\alpha$')
plt.ylabel(r'$frac{d\phi}{d\alpha}$')
plt.legend()


Visually, these are indistinguishable from each other. We can look at the errors too, and here we see they are negligible, and probably we can attribute them to the approximation we use for the integral, and not due to automatic differentiation.

perr = (analytical_dphi(alpha) - dphidalpha) / analytical_dphi(alpha) * 100
plt.plot(alpha, perr, label='analytical')
plt.xlabel(r'$\alpha$')
plt.ylabel('%error')


## 2 Example 2

In example 2 there is this function, which has variable limits:

$$f(x) = \int_{\sin x}^{\cos x} \cosh t^2 dt$$

What is $$f'(x)$$ here? It can be derived with some effort and it is:

$$f'(x) = -\cosh(\cos^2 x) \sin x - \cosh(\sin^2 x) \cos x$$

This function was kind of fun to code up, I hadn't thought about how to represent variable limits, but here it is.

def f(x):
a = np.sin(x)
b = np.cos(x)
t = np.linspace(a, b, 1000)
y = np.cosh(t**2)
return trapz(y, t)

# Here is our derivative!


Here is a graphical comparison of the two:

x = np.linspace(0, 2 * np.pi)

analytical = -np.cosh(np.cos(x)**2) * np.sin(x) - \
np.cosh(np.sin(x)**2) * np.cos(x)
ad = [dfdx(_x) for _x in x]

plt.plot(x, analytical, label='analytical')
plt.xlabel('x')
plt.ylabel('df/dx')
plt.legend()


These are once again indistinguishable.

## 3 Summary

These are amazing results to me. Before trying it, I would not have thought it would be so easy to evaluate the derivative of these functions. These work of course because all the operations involved in computing the integral are differentiable and defined in autograd. It certainly opens the door to all kinds of new approaches to solving engineering problems that need the derivatives for various purposes like optimization, sensitivity analysis, etc.

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## Compressibility variation from an implicit equation of state

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In this post I explored using automatic differentiation to compute how the compressibility of a gas defined by the van der Waal equation varies with the reduced pressure. In that example we had an explicit function of the pressure as a function of the volume and temperature, and we could derive a differential equation that defines the variation we were interested in.

I thought we should be able to derive the differential equation more directly, still using automatic differentiation and we explore that idea here. The general strategy to compute the compressibility as a function of pressure is to integrate $$dV / dP_r$$ over a range of $$P_r$$ to get the molar volume as a function of $$P_r$$, and then to directly compute the compressibility from $$Z = PV/(RT)$$.

To use this approach we need to get $$dV / dP_r$$ from the van der Waal equation. Previously, we derived this in a round about way from the explicit form of the van der Waal equation. Here, we follow the work in this post to get the derivative from the implicit form of the van der Waal equation:

$$f(V, P_r, T_r) = \frac{R Tr * Tc}{V - b} - \frac{a}{V^2} - P_r Pc = 0$$

Based on the work in this post, we can get

$$dV/dP_r = (-df/dP_r) / (df/dV)$$

and the two derivatives on the right can be found easily by automatic differentiation. First, we express the van der Waal equation in implicit form, with the variables as $$V, P_r, T_r$$. Only two of those variables are independent; if you define two of them you can compute the third one using a tool like fsolve.

R = 0.08206
Pc = 72.9
Tc = 304.2

a = 27 * R**2 * Tc**2 / (Pc * 64)
b = R * Tc / (8 * Pc)

Tr = 1.1  # Constant for this example

def f(V, Pr, Tr):
return R * Tr * Tc / (V - b) - a / V**2 - Pr * Pc


Now, if we want to know how does the volume vary with $$P_r$$, we need to derive the derivative $$dV/dP_r$$, and then integrate it. Here we use autograd to define the derivatives, and then we define a function that uses them. Note the arguments in the function dVdPr are in an order that anticipates we want to integrate it in solve_ivp, to get a function $$V(P_r)$$.

from autograd import grad

dfdPr = grad(f, 1)  # derivative of f with respect to arg at index=1: Pr
dfdV = grad(f, 0)  # derivative of f with respect to arg at index=0: V

def dVdPr(Pr, V):
return -dfdPr(V, Pr, Tr) / dfdV(V, Pr, Tr)  # Tr is a constant in here


Now, we need an initial condition to start the integration from. We want the volume at $$P_r=0.1$$. We have to use fsolve for this, or some other method that tells you want is the volume at $$P_r=0.1$$. We can pass the values of $$P_r$$ and $$T_R$$ as arguments to our implicit function. Since $$V$$ is the first argument, we can directly solve our implicit function. Otherwise you would have to define a helper objective function to use with fsolve.

from scipy.optimize import fsolve

V0, = fsolve(f, 3.5, args=(0.1, 1.1))
V0

3.6764763125625435



Finally, we are ready to integrate the ODE, and plot the solution.

import numpy as np
from scipy.integrate import solve_ivp

Pr_span = (0.1, 10)
Pr_eval, h = np.linspace(*Pr_span, retstep=True)

sol = solve_ivp(dVdPr, Pr_span, (V0,), max_step=h)
print(sol.message)

%matplotlib inline
import matplotlib.pyplot as plt

Pr = sol.t  # the P_r steps used in the solution
V = sol.y[0]  # V(P_r) from the solution

Z = Pr * Pc * V / (R * Tr * Tc)  # Compressibility Z(P_r)

plt.plot(Pr, Z)
plt.xlabel('$P_r$')
plt.ylabel('Z')
plt.xlim([0, 10])
plt.ylim([0, 2])

The solver successfully reached the end of the integration interval.


(0, 2)



That is the same result as we got before.

## 1 Summary thoughts

This method also worked successfully to solve this problem. In most ways, this method has less algebraic manipulations required to get to the solution. In method 3, we had to do some calculus that relied on a particular explicit form of the van der Waal equation. While those manipulations were not particularly difficulty, the leave opportunities for mistakes, and they will be more difficult for an implicit equation of state (e.g. if there was a $$P$$ on the right hand side).

This approach also required some manipulation, but it is a standard one and that is how do you get a derivative from an implicit function. After that, it is straightforward to define the desired derivative as a function and then integrate it to get the solution. So, we still don't get a free pass on calculus, but we do reduce the number of manipulations required to get to the solution. I consider that a plus.

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## Getting derivatives from implicit functions with autograd

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If we have an implicit function: $$f(x, y(x)) = 0$$, but we want to compute the derivative $$dy/dx$$ we can use the chain rule to derive:

$$df/dx + df/dy dy/dx = 0$$

We can then solve for $$dy/dx$$:

$$dy/dx = -df/dx / df/dy$$ to get the desired derivative. The interesting point of this blog post is that we can get the two derivatives on the right hand side of this equation using automatic differentiation of the function $$f(x, y)$$! There are a few examples of analytical approaches to derivatives from implicit functions here, and I wanted to explore them with autograd in this post.

In the following examples, we will assume that $$y$$ is a function of $$x$$ and that $$x$$ is independent. We will consider a series of implicit equations, compute $$dy/dx$$ using autograd from the formula above, and compare them to the analytical results in the web page referenced above.

The $$dy/dx$$ functions generally depend on both $$x$$, and $$y$$. Technically, these are the derivatives along the curve $$y(x)$$, but since we can evaluate them at any points, we will use some random points for $$x$$ and $$y$$ to test for equality between the analytical derivatives and the autograd derivatives. This isn't a rigorous proof of equality, but it is the only thing that makes sense to do for now. It is assumed that if these points are ok, all the others are too. That might be a broad claim, since we only sample $$x$$ and $$y$$ from 0 to 1 here but the approach is general. Here are the imports and the random test points for all the examples that follow.

import autograd.numpy as np

xr = np.random.random(50)
yr = np.random.random(50)


## 1 Example 1

$$x^3 + y^3 = 4$$

def f1(x, y):
return x**3 + y**3 - 4

dydx_1 = lambda x, y: -D1x(x, y) / D1y(x, y)
dydx_1a = lambda x, y: -x**2 / y**2

np.allclose(dydx_1a(xr, yr),
[dydx_1(_xr, _yr) for _xr, _yr in zip(xr, yr)])

True



The output of True means the autograd results and the analytical results are "all close", i.e. within a tolerance the results are the same. The required derivatives of this are not that difficult to derive, but it is nice to see a simple example that "just works". A subtle point of the dydx function is that it is not vectorized which is why I used a list comprehension to evaluate all the points. It might be possible to make a pseudo-vectorized version with the np.vectorize decorator, but that is not of interest here.

## 2 Example 2

$$(x - y)^2 = x + y - 1$$

def f2(x, y):
return (x - y)**2 - x - y + 1

dydx_2 = lambda x, y: -D2x(x, y) / D2y(x, y)
dydx2_a = lambda x, y: (2 * y - 2 * x + 1) / (2 * y - 2 * x - 1)

np.allclose(dydx2_a(xr, yr),
[dydx_2(_xr, _yr) for _xr, _yr in zip(xr, yr)])

True



This also works.

## 3 Example 3

$$y = sin(3x + 4y)$$

def f3(x, y):
return y - np.sin(3 * x + 4 * y)

dydx_3 = lambda x, y: -D3x(x, y) / D3y(x, y)
dydx3_a = lambda x, y: (3 * np.cos(3 * x + 4 * y)) / (1 - 4 * np.cos(3 * x + 4 * y))

np.allclose(dydx3_a(xr, yr),
[dydx_3(_xr, _yr) for _xr, _yr in zip(xr, yr)])

True



Check.

## 4 Example 4

$$y = x^2 y^3 + x^3 y^2$$

def f4(x, y):
return y - x**2 * y**3 - x**3 * y**2

dydx_4 = lambda x, y: -D4x(x, y) / D4y(x, y)
dydx4_a = lambda x, y: (2 * x * y**3 + 3 * x**2 * y**2) / (1 - 3 * x**2 * y**2 - 2 * x**3 * y)

np.allclose(dydx4_a(xr, yr),
[dydx_4(_xr, _yr) for _xr, _yr in zip(xr, yr)])

True



## 5 Example 5

$$e^{xy} = e^{4x} - e^{5y}$$

def f5(x, y):
return np.exp(4 * x) - np.exp(5 * y) - np.exp(x * y)

dydx_5 = lambda x, y: -D5x(x, y) / D5y(x, y)
dydx5_a = lambda x, y: (4 * np.exp(4 * x) - y * np.exp(x * y)) / (x * np.exp(x * y) + 5 * np.exp(5 * y))

np.allclose(dydx5_a(xr, yr),
[dydx_5(_xr, _yr) for _xr, _yr in zip(xr, yr)])

True



Also check.

## 6 Example 6

$$\cos^2 x + cos^2 y = cos(2x + 2y)$$

def f6(x, y):
return np.cos(x)**2 + np.cos(y)**2 - np.cos(2 * x + 2 * y)

dydx_6 = lambda x, y: -D6x(x, y) / D6y(x, y)
dydx6_a = lambda x, y: (np.cos(x) * np.sin(x) - np.sin(2 * x + 2 * y)) / (np.sin(2 * x + 2 * y) - np.cos(y) * np.sin(y))

np.allclose(dydx6_a(xr, yr),
[dydx_6(_xr, _yr) for _xr, _yr in zip(xr, yr)])

True



Check.

## 7 Example 7

$$x = 3 + \sqrt{x^2 + y^2}$$

def f7(x, y):
return 3 + np.sqrt(x**2 + y**2) - x

dydx_7 = lambda x, y: -D7x(x, y) / D7y(x, y)
dydx7_a = lambda x, y: (np.sqrt(x**2 + y**2) - x) / y

np.allclose(dydx7_a(xr, yr),
[dydx_7(_xr, _yr) for _xr, _yr in zip(xr, yr)])

True



## 8 Conclusions

There are a handful of other examples at the website referenced in the beginning, but I am stopping here. After seven examples of quantitative agreement between the easy to derive autograd derivatives, and the easy to moderately difficult analytical derivatives, it seems like it is autograd for the win here. This technique has some important implications for engineering calculations that I will explore in a future post. Until then, this is yet another interesting thing we can do with automatic differentiation!

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## Compressibility factor variation from the van der Waals equation by three different approaches

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In the book Problem solving in chemical and biochemical engineering with POLYMATH, Excel and Matlab by Cutlip and Shacham there is a problem (7.1) where you want to plot the compressibility factor for CO2 over a range of $$0.1 \le P_r <= 10$$ for a constant $$T_r=1.1$$ using the van der Waal equation of state. There are a two standard ways to do this:

1. Solve a nonlinear equation for different values of $$P_r$$.
2. Solve a nonlinear equation for one value of $$P_r$$, then derive an ODE for how the compressibility varies with $$P_r$$ and integrate it over the relevant range.

In this post, we compare and contrast the two methods, and consider a variation of the second method that uses automatic differentiation.

## 1 Method 1 - fsolve

The van der Waal equation of state is:

$$P = \frac{R T}{V - b} - \frac{a}{V^2}$$.

We define the reduced pressure as $$P_r = P / P_c$$, and the reduced temperature as $$T_r = T / T_c$$.

So, we simply solve for V at a given $$P_r$$, and then compute $$Z$$. There is a subtle trick needed to make this easy to solve, and that is to multiply each side of the equation by $$(V - b)$$ to avoid a singularity when $$V = b$$, which happens in this case near $$P_r \approx 7.5$$.

from scipy.optimize import fsolve
import numpy as np
%matplotlib inline
import matplotlib.pyplot as plt

R = 0.08206
Pc = 72.9
Tc = 304.2

a = 27 * R**2 * Tc**2 / (Pc * 64)
b = R * Tc / (8 * Pc)

Tr = 1.1

def objective(V, Pr):
P = Pr * Pc
T = Tr * Tc
return P * (V - b) - (R * T)  +  a / V**2 * (V - b)

Pr_range = np.linspace(0.1, 10)
V = [fsolve(objective, 3, args=(Pr,))[0] for Pr in Pr_range]

T = Tr * Tc
P_range = Pr_range * Pc
Z = P_range * V / (R * T)

plt.plot(Pr_range, Z)
plt.xlabel('$P_r$')
plt.ylabel('Z')
plt.xlim([0, 10])
plt.ylim([0, 2])

(0, 2)



That looks like Figure 7-1 in the book. This approach is fine, but the equation did require a little algebraic finesse to solve, and you have to use some iteration to get the solution.

## 2 Method 2 - solve_ivp

In this method, you have to derive an expression for $$\frac{dV}{dP_r}$$. That derivation goes like this:

$$\frac{dV}{dP_r} = \frac{dV}{dP} \frac{dP}{dP_r}$$

The first term $$\frac{dV}{dP}$$ is $$(\frac{dP}{dV})^{-1}$$, which we can derive directly from the van der Waal equation, and the second term is just a constant: $$P_c$$ from the definition of $$P_r$$.

They derived:

$$\frac{dP}{dV} = -\frac{R T}{(V - b)^2} + \frac{2 a}{V^3}$$

We need to solve for one V, at the beginning of the range of $$P_r$$ we are interested in.

V0, = fsolve(objective, 3, args=(0.1,))
V0

3.6764763125625461



Now, we can define the functions, and integrate them to get the same solution. I defined these pretty verbosely, just for readability.

from scipy.integrate import solve_ivp

def dPdV(V):
return -R * T / (V - b)**2 + 2 * a / V**3

def dVdP(V):
return 1 / dPdV(V)

dPdPr = Pc

def dVdPr(Pr, V):
return dVdP(V) * dPdPr

Pr_span = (0.1, 10)
Pr_eval, h = np.linspace(*Pr_span, retstep=True)

sol = solve_ivp(dVdPr, Pr_span, (V0,), dense_output=True, max_step=h)

V = sol.y[0]
P = sol.t * Pc
Z = P * V / (R * T)
plt.plot(sol.t, Z)
plt.xlabel('$P_r$')
plt.ylabel('Z')
plt.xlim([0, 10])
plt.ylim([0, 2])

(0, 2)



This also looks like Figure 7-1. It is arguably a better approach since we only need an initial condition, and after that have a reliable integration (rather than many iterative solutions from an initial guess of the solution in fsolve).

The only downside to this approach (in my opinion) is the need to derive and implement derivatives. As equations of state get more complex, this gets more tedious and complicated.

## 3 Method 3 - autograd + solve_ivp

The whole point of automatic differentiation is to get derivatives of functions that are written as programs. We explore here the possibility of using this to solve this problem. The idea is to use autograd to define the derivative $$dP/dV$$, and then solve the ODE like we did before.

from autograd import grad

def P(V):
return R * T / (V - b) - a / V**2

def dVdPr(Pr, V):
return 1 / dPdV(V) * Pc

sol = solve_ivp(dVdPr,  Pr_span, (V0,), dense_output=True, max_step=h)

V, = sol.y
P = sol.t * Pc
Z = P * V / (R * T)
plt.plot(sol.t, Z)
plt.xlabel('$P_r$')
plt.ylabel('Z')
plt.xlim([0, 10])
plt.ylim([0, 2])

(0, 2)



Not surprisingly, this answer looks the same as the previous ones. I think this solution is pretty awesome. We only had to implement the van der Waal equation, and then let autograd do its job to get the relevant derivative. We don't get a free pass on calculus here; we still have to know which derivatives are important. We also need some knowledge about how to use autograd, but with that, this problem becomes pretty easy to solve.

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## Solving nonlinear algebra problems with internal state information

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In engineering, we often need to solve an equation in one variable, and then use the solution to compute other variables. For example, we might want the bubble point temperature of a mixture, and then to determine the composition of the vapor phase that has formed. In other words, we compute the temperature, and then have to use it in a subsequent step to get the composition. Here is a bubble point computation adapted from example 10.2 in Smith and van Ness, Introduction to Chemical Engineering Thermodynamics.

Given a solution of acetone (x1=0.3), acetonitrile (x2=0.45) and nitromethane (x3=0.25) at a total pressure of 80 kPa, compute the bubble point temperature and gas phase composition.

The key here is to find a temperature where the gas-phase mole fractions sum to one. The gas phase mole fractions are defined by:

$$y_i = x_i Pvap_i(T) / P$$

The typical way I would teach students how solve this looks like this. It uses the Antoine equation coded below to estimate the vapor pressure of each component as a function of temperature, and then uses fsolve to find a temperature where the gas-phase mole fractions sum to one.

import numpy as np
from scipy.optimize import fsolve

acetone = (14.5463, 2940.46, 237.22)
acetonitrile = (14.2724, 2945.47,224)
nitromethane = (14.2043, 2972.64, 209)

def antoine(T, A, B, C):
T = float(T) # there is some subtle issue that comes up when T is an array,
# as passed in from fsolve. It needs to be a float, or you get
return np.exp(A - B / (T + C))

x = np.array([0.3, 0.45, 0.25])
P = 80

def objective(T):
Pvap = np.array([antoine(T, *pars) for pars in [acetone, acetonitrile, nitromethane]])
y = x * Pvap / P
return 1 - y.sum()

Tans, = fsolve(objective, 70)

# This is where we end up repeating code
Pvap = np.array([antoine(Tans, *pars) for pars in [acetone, acetonitrile, nitromethane]])
y = x * Pvap / P

print(f'The bubble point temperature is {Tans:1.2f} degC, and the gas phase compositions are {np.round(y, 4)}.')

The bubble point temperature is 68.60 degC, and the gas phase compositions are [ 0.5196  0.3773  0.1031].



This solution works fine, but there is in my opinion, an issue with the small amount of repeated code at the end that is required to get the composition of the gas-phase. This is a small problem here, but as the problems get bigger it is more and more tedious to correctly repeat all the code to see what the state of a system is at the solution, and it seems wasteful to have to repeat the computations; they were known in the objective function. In the following subsections, I explore some alternative approaches to reduce the repetition.

## 1 First approach

There are two small chunks of repeated code in the example above. One way to minimize the amount of repeated code is to pull these out into reusable functions. Here, we do that, and only have to repeat one function call at the end to get the system composition out.

import numpy as np
from scipy.optimize import fsolve

acetone = (14.5463, 2940.46, 237.22)
acetonitrile = (14.2724, 2945.47,224)
nitromethane = (14.2043, 2972.64, 209)

def antoine(T, A, B, C):
T = float(T) # there is some subtle issue that comes up when T is an array,
# as passed in from fsolve. It needs to be a float, or you get
return np.exp(A - B / (T + C))

x = np.array([0.3, 0.45, 0.25])
P = 80

def Pvap(T):
return np.array([antoine(T, *pars) for pars in [acetone, acetonitrile, nitromethane]])

def y(T):
return x * Pvap(T) / P

def objective(T):
return 1 - y(T).sum()

Tans, = fsolve(objective, 70)

yans = y(Tans) # minimal repetition of a calculation to get the composition state.

print(f'The bubble point temperature is {Tans:1.2f} degC, and the gas phase compositions are {np.round(yans, 4)}.')

The bubble point temperature is 68.60 degC, and the gas phase compositions are [ 0.5196  0.3773  0.1031].



That is a small improvement. The code is not much shorter, just reorganized for easier reuse. It would be easy in this case to also get the vapor pressures of each species at this temperature, just by calling the Pvap function. Still, it feels annoying we have to recalculate the answer to something you know must have already been known when the objective function was evaluated.

## 2 Second approach - use a state dictionary as an arg in the objective function

In this approach, we will use a dictionary to store the state of the objective function. The dictionary will be in the global namespace, and we will just update it each time the objective function is called.

import numpy as np
from scipy.optimize import fsolve

acetone = (14.5463, 2940.46, 237.22)
acetonitrile = (14.2724, 2945.47,224)
nitromethane = (14.2043, 2972.64, 209)

def antoine(T, A, B, C):
return np.exp(A - B / (T + C))

x = np.array([0.3, 0.45, 0.25])

state = {}

P = 80

def objective(T, state):
T = float(T)
Pvap = np.array([antoine(T, *pars) for pars in [acetone, acetonitrile, nitromethane]])
y = x * Pvap / P
state.update({'y': y,
'T':  T,
'Pvap': Pvap,
'z': 1 - y.sum()})
return state['z']

Tans, = fsolve(objective, 70, args=(state,))

print(f'The bubble point temperature is {Tans:1.2f} degC, and the gas phase compositions are {np.round(state["y"], 4)}.')
print(Tans- state['T']) # check to make sure last value from objective is the same as the solution
state

The bubble point temperature is 68.60 degC, and the gas phase compositions are [ 0.5196  0.3773  0.1031].
0.0


{'Pvap': array([ 138.5620209 ,   67.07966082,   32.98218545]),
'T': 68.60064626680659,
'y': array([ 0.51960758,  0.37732309,  0.10306933]),
'z': -3.4194869158454821e-14}



What we see in the state dictionary is the result from the last time that the objective function was called. It appears that the list time it was called is also where the solution comes from, so the other variables stored here should be consistent. Now you can see we have access to both the Pvap and y composition data from the solution without needing any further computations. This approach could be easily extended to store any derived quantities that represent internal states you want. We don't store any history in this, but you could by appending to lists in the dictionary.

One feature of this is the state dictionary is updated by side effect, and you have to use the state dictionary as an argument parameter to the function.

## 3 third approach - a callable object

A standard approach to tracking state data is to encapsulate it in a class. fsolve requires a callable function that returns zero at the solution. It is possible to make an object act like a callable function if we define a __call__ method on it. Then, in this method, we can set attributes on the object to keep track of the state, similar to what we did with the dictionary. Since we have a class, we can define some other special dunder methods, e.g. to print the solution. Here is one implementation.

import numpy as np
from scipy.optimize import fsolve

class Objective(object):
acetone = (14.5463, 2940.46, 237.22)
acetonitrile = (14.2724, 2945.47,224)
nitromethane = (14.2043, 2972.64, 209)

def __init__(self, x, P):
self.x = np.array(x)
self.P = P

def antoine(self, T, A, B, C):
return np.exp(A - B / (T + C))

def __str__(self):
s = f'The bubble point temperature is {self.T:1.2f} degC, and the gas phase compositions are {np.round(self.y, 4)}.'
return s

def __call__(self, T):
T = float(T)
self.T = T
self.Pvap = np.array([self.antoine(T, *pars) for pars in [self.acetone, self.acetonitrile, self.nitromethane]])
self.y = self.x * self.Pvap / self.P
return 1 - self.y.sum()

obj = Objective(x=np.array([0.3, 0.45, 0.25]), P=80)
ans, = fsolve(obj, 60)

print(obj)

The bubble point temperature is 68.60 degC, and the gas phase compositions are [ 0.5196  0.3773  0.1031].



Similar to the state dictionary approach, there is no repeated code here, and no repeated evaluations to get to the state after the solution. This is a bit more advanced Python than the state dictionary. Note, this implementation doesn't have any checking in it, so if you try to print the object before calling fsolve, you will get an error because the attributes don't exist until after the object has been called. That is also an issue with the state dictionary above.

There are many choices you could make in constructing this example. Maybe this one has gone too far in encapsulating the antoine function as a method. That limits its reusability for another problem. On the other hand, you can reuse it for any other pressure or liquid composition of acetone, acetonitrile and nitromethane very readily.

## 4 Summary

We looked at three ways to reduce having redundant code in the solution to problems involving nonlinear algebra. The first approach is conceptually simple; you break out as much as you can into reusable functions, and then at most have repeated function calls. These computations are usually not expensive, so repeating them is mostly tedious and provides opportunities for mistakes. This is probably what I will stick to for teaching students that are just seeing this for the first time.

The second approach used a dictionary (other data structures could work too) as an argument to the objective function, and internal states were kept in the dictionary so that after the problem was solved, you have immediate access to them. This is more advanced than the first approach because it requires understanding that the dictionary is modified as a side effect of solving the problem.

Finally, we considered an object-oriented class encapsulation of the information we wanted. I consider this the most advanced Python solution, since it requires some understanding of classes, dunder methods and attributes, and how to make an instance of a class.

The last two methods seem like candidates for an advanced class in problem solving. Thoughts?